1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to a new sandwich cathode design having a second cathode active material of a relatively high energy density but of a relatively low rate capability sandwiched between two current collectors and with a first cathode active material having a relatively low energy density but of a relatively high rate capability in contact with the opposite sides of the current collectors. The present cathode design is useful for powering an implantable medical device requiring a high rate discharge application.
2. Prior Art
Early ventricular cardiac defibrillators used two lithium electrochemical cells, in series, as their power source. However, the electronic circuit in current cardiac defibrillators now consume less energy than earlier models. This makes it possible for defibrillators today to use a single lithium cell as the power source. With a one cell design, the requirement for high current pulse capability, or power density, is even greater due to lowered pulsing voltage. A large electrode surface area is thus needed to accomplish this requirement. However, when the electrode surface area is increased, more inert materials (current collector, separator, etc.) are introduced into the cell. As a result, the cell""s volumetric capacity is decreased. Another concern is the longevity of the medical device, which is dependent on the cell""s capacity and power efficiency.
The capacity of an electrochemical cell is not only dependent on the electrode assembly design and packing efficiency, it also is dependent on the type of active materials used. For example, it is generally recognized that for lithium cells, silver vanadium oxide (SVO) and, in particular, xcex5-phase silver vanadium oxide (AgV2O5.5), is preferred as the cathode active material. This active material has a theoretical volumetric capacity of 1.37 Ah/ml. By comparison, the theoretical volumetric capacity of CFx material (x=1.1) is 2.42 Ah/ml, which is 1.77 times that of xcex5-phase silver vanadium oxide. However, for powering a cardiac defibrillator, SVO is preferred because it can deliver high current pulses or high energy within a short period of time. Although CFx has higher volumetric capacity, it cannot be used in medical devices requiring a high rate discharge application due to its low to medium rate of discharge capability.
The attempt to use high capacity materials, such as CFx, by mixing it with a high rate cathode material, such as SVO, is reported in U.S. Pat. No. 5,180,642 to Weiss et. al. However, electrochemical cells made from such cathode composites have lower rate capability. The benefit of increasing the cell theoretical capacity by using CFx as part of the cathode mix is in part canceled by the lowering of its power capability in a high rate discharge application.
Another way to address the longetivity issue is described in U.S. Pat. No. 5,614,331 to Takeuchi et al., which is assigned to the assignee of the present invention and incorporated hereby by reference. In this patent, a method of using a medium rate CFx cell to power the circuitry of an implantable defibrillator while simultaneously using a SVO cell to provide the power supply under high rate application for the device is described. The advantage of this method is that all of the high power SVO energy is reserved for the high power application such as charging a capacitor while the device monitoring function, for example for monitoring the heart beat, which require generally low power requirements, is provided by the high capacity CFX system. However, this battery construction requires a very careful design to balance the capacities of the high power cell (SVO) and the low power cell (CFx) with both cells reaching end of service life at or near the same time. Such a balance, nevertheless, is very difficult to achieve due to the variable device usage requirements of a particular patient.
As is well known by those skilled in the art, an implantable cardiac defibrillator is a device that requires a power source for a generally medium rate, constant resistance load component provided by circuits performing such functions as, for example, the heart sensing and pacing functions. From time-to-time, the cardiac defibrillator may require a generally high rate, pulse discharge load component that occurs, for example, during charging of a capacitor in the defibrillator for the purpose of delivering an electrical shock to the heart to treat tachyarrhythmias, the irregular, rapid heartbeats that can be fatal if left uncorrected.
Accordingly, the object of the present invention is to improve the performance of lithium electrochemical cells by providing a new concept in electrode design. Further objects of this invention include providing a cell design for improving the capacity and utilization efficiency of defibrillator batteries, and to maintain the high current pulse discharge capability throughout the service life of the battery.
To fulfill these needs, a new sandwich cathode design is provided having a first cathode active material of a relatively high energy density but of a relatively low rate capability, for example CFx, sandwiched between two current collectors and with a second cathode active material having a relatively low energy density but of a relatively high rate capability, for example SVO, in contact with the opposite sides of the current collectors. Such an exemplary cathode design might look like SVO/current collector/CFx/current collector/SVO.